In recent years, one function that has been installed in storage apparatuses is a virtualization function known as thin provisioning. The thin provisioning function is a function which provides a virtual logical volume (hereinafter called a virtual volume) to a host computer and which, when a request to write data to the virtual volume is supplied from the host computer, dynamically assigns a storage area to the virtual volume. The thin provisioning function affords the benefit of making it possible to provide a virtual volume with a larger capacity than the storage area which can actually be provided to the host computer and making it possible to construct a computer system at low cost by reducing the physical storage capacity in the storage apparatus which is to be pre-prepared.
Furthermore, as a data management method for a storage apparatus which has such a thin provisioning function installed, a hierarchical data management method has conventionally been proposed. The hierarchical data management method manages each of the storage areas provided by storage devices of a plurality of types of varying performance which are installed in the storage apparatus as storage tiers of a plurality of different types, assigns storage areas from high-speed and high-performance storage tiers to areas storing high access frequency data in the virtual volume, and assigns storage areas from low-speed and low-performance storage tiers to areas storing low access frequency data in the virtual volume. With this kind of hierarchical data management method, the cost performance of the storage apparatus can be improved.
Furthermore, types of storage apparatuses include file storage apparatuses which handle data in file units and block storage apparatuses which handle data in block units (page units), where the file storage apparatuses realize file-unit hierarchical control and the block storage apparatuses realize block-unit hierarchical control (PTL1 and PTL2, for example).